Research Articles (Soil, Crop and Climate Sciences)
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Item Open Access Downscaling legacy soil information for hydrological soil mapping using multinomial logistic regression(Elsevier, 2023) Smit, I.E.; Van Zijl, G.M.; Riddell, E.S.; Van Tol, J.J.In South Africa, there is a growing demand for large scale detailed hydrological soil maps for modelling and management purposes. However, imbalanced legacy soil information often impedes the accurate creation of such maps by not being representative of the environmental complexity of large-scale catchments and containing imbalanced soil class distributions, often resulting in the loss of minority soil classes, which are often of great hydrological importance (e.g., wetland and riparian soils). In this study, we proposed a new downscaling approach to handle spatially localised legacy soil data within a larger low resolution legacy soil dataset to create an accurate hydrological soil map of the macro-scale (5790 km2) Sabie-Sand catchment using multinomial logistic regression (MNLR). The spatially localised legacy data was downscaled using k-means clustering and added to the broader legacy dataset. Five levels of legacy soil data were analysed in their representation of environmental covariates using QQ-plots and a Welsh’s t-test and their mapping accuracy using confusion matrix’s and Kappa coefficient statistics. However, MNLR also requires balanced soil classes. The value of the best performing legacy soil dataset was also compared to using all available soil information after both had their soil class distributions fully balanced using Synthetic Minority Oversampling Technique (SMOTE). The 500 ha/observation-SMOTE dataset resulted in the most accurate hydrological soil map with a validation point accuracy of 73% and a Kappa coefficient of 0.60, substantially outperforming the other downscaled soil maps as well as the SMOTE balanced dataset using all available soil information. This was due to the decreased variation between observations and catchment means, where the 500 ha/observation dataset yielded the least variation between soil observation and catchment datasets and well as reducing the class imbalance within the legacy soil data. Downscaling spatially localised legacy soil data for environmental representation is an effective tool to improve digital soil mapping accuracy using MNLR.Item Open Access Soil fertility status under mixed pastures in irrigated Tsitsikamma dairy farms: case studies(Springer, 2024) Loke, P. F.; Kotzé, E.; Du Preez, C. C.Animal manures are increasingly receiving renewed interest as an alternative to synthetic fertilizers. While they may improve ecosystem functions, there is limited information on short-term effects of organic amendments on soil reaction and nutrient dynamics in irrigated mixed dairy pastures, especially in the context of the Tsitsikamma region, South Africa. This study examined the soil fertility status of minimum tilled 6-year-old pasture-mixed dairy farms (F1, F2, F3, F4 and F5) in the Upper (UT) and Lower (LT) Tsitsikamma regions treated with different rates of NPK (nitrogen, phosphorus, potassium) fertilizer alone or in combination with poultry manure (PM) and/or dairy effluent (DE). Soil samples were collected at 0–15, 15–30, 30–45 and 45–60 cm depth intervals and analysed for soil pH, P, K, Ca and Mg. Results of this study revealed variable trends on soil pH and nutrient changes between farms, suggesting an influence of some inherent soil properties in addition to the 6-year applied management practices. When averaged over sampled farms, surface placement of soil amendments and limited soil disturbance resulted in surface stratification of soil pH, P, K, Ca and Mg. On the other hand, integration of organic and inorganic fertilizers induced significant changes in nutrient contents and stocks to a depth of 60 cm, especially in the LT region. A combination of NPK fertilizer, PM and/or DE applied in pasture mixtures generally showed potential to improve soil fertility in both regions. As such, adoption of this combination by farmers could cut down reliance on expensive synthetic fertilizers and costs of dairy production. However, studies are still necessary in this region to validate the observed results.Item Open Access Firebreaks and their effect on vegetation composition and diversity in grasslands of Golden Gate Highlands National Park, South Africa(MDPI, 2024) van Aardt, Andri C.; vde Jager, J. C. Linde; van Tol, Johan J.Southern African grasslands with a rich flora, shaped by fire, grazing, climate and geology, as well as playing a role in carbon sequestration, are becoming more important in conservation. Fire is often used as a management tool to improve vegetation and to protect property against uncontrolled fire. We therefore attempt to determine the effect consecutive burning has on vegetation. Paired plots along firebreaks were used to collect vegetation data using the Braun-Blanquet cover abundance scale. Soil samples were also collected to determine the impact of fire on below-ground nitrogen (N) and carbon (C) stocks and ratios. The results indicate that there is no difference between the plant communities of the firebreaks and the adjacent grassland; however, there are certain species that are favoured by firebreaks and others by the adjacent grassland. There is also no difference in diversity between the firebreaks and adjacent grassland areas. Carbon and nitrogen stocks as well as C:N ratios did not differ significantly between the firebreaks and the adjacent grassland plots although trends indicate a decline in both C and N with repeated burning.Item Open Access Using an uptake enhancer to mitigate nitrogen leaching while enhancing uptake efficiency(MDPI, 2024) Somi, Zoyolo; Kotzé, Elmarie; Van der Watt, ElmarieNitrogen (N) has the most crucial influence on raising agricultural productivity of all other plant nutrients given to crops. However, 50% of the N given to crops is dissipated to the environment globally, resulting in environmental concerns due to leaching. Current research shows that intensive agricultural production systems, which are still used in a large proportion around the world, are prone to N loss. This study aimed to investigate the effect of uptake enhancer applications on N movement in the soil profile based on 10 cm depth intervals, as well as its effects on N uptake and vegetative growth of oats at 4-week intervals over a 16-week period, using sandy soil as a growing medium. Oats were cultivated in a glasshouse setting in polyvinyl chloride (PVC) columns of 60 cm in height. Six treatments were employed at the 3rd leaf growth stage, and each was replicated four times. The experiment had a constructive and a destructive part, which was employed to monitor crop N uptake at four growth stages. Analyses of soil and plant samples were carried out in all the growth stages. The treatments containing the uptake enhancer prevented N from leaching, particularly at the top 20 cm soil depth, with impressive reductions of 194% at 0–10 cm depth and 186% at 10–20 cm depth, during the first 4 weeks after planting. The uptake enhancer also promoted early vegetative growth and crop performance with 15%. In conclusion, the study revealed that employing the uptake enhancer can improve the efficacy of N fertilizer, thereby reducing the application rate of the fertilizer in agroecosystems.Item Open Access Heat pulse velocity method for determining water requirements in rainfed sweet cherry trees (Prunus avium L.)(Frontiers, 2023) Tharaga, Phumudzo C.; Tesfuhuney, Weldemichael A.; Coetzer, Gesine M.; Savage, Michael J.It is imperative to possess a comprehensive understanding of the water consumption patterns of sweet cherry trees under rainfed conditions. This study investigates the water use of sweet cherry using the heat pulse velocity sap flow method. Tree response to drying soil conditions over two consecutive measurement periods (September 2017 to December 2018) in the eastern Free State, South Africa, is also evaluated. During the post-harvest period (December), there was a continuous increase in daily water consumption by trees, in conjunction with an increase in canopy cover, even though the crop load was reduced. Measured sap flow was positively correlated to net radiation, air temperature, and water vapour pressure deficit. The transpiration rates ranged from 1.2 to 3.5 L d-1 during the flowering stage (day of year, DOY, 244 – 270) and showed an increasing trend as the days progressed. During the ripening stage (DOY 271 – 292), transpiration rates decreased from 4.5 to 1.1 L d-1 over the 2018 season. This decreasing trend from the previous growth stage was due to soil drying and scorching weather conditions that led to trees experiencing water stress. Sap flow measurements, however, showed typical characteristics of the diurnal trend during selected days during varying weather conditions. The fraction of transpirable soil water (FTSW) threshold varied for different fruit growth stages and approached 1.0 for different stress levels. FTSW exceeded 0.4 when sweet cherry trees utilised stored soil water, while the transpiration rates declined during prolonged hot days. Moreover, the stress coefficient factor ranged between 0.45 – 0.65 for different growth stages. The daily soil water content varied, and soil evaporation was expected to increase during hot and dry days. In the early stage of a dry spell, soil water content did not directly affect the transpiration rate. Sweet cherry trees are susceptible to soil water deficit at different stages of fruit development. More research is required to understand transpiration as an irrigation management and planning indicator.Item Open Access Potential distribution of selected invasive alien plants under current and future climate change scenarios in South Africa(Elsevier, 2023) Mengistu, Achamyeleh G.; Tesfuhuney, Weldemichael A.; Woyessa, Yali E.; Steyn, Abraham S.Invasive alien plants are one of the main causes for the decline of native biodiversity worldwide. Hence, it is crucial to understand the dynamics of invasive plants in the context of a changing climate. The main aim of this study was to evaluate the potential distribution of two major invasive alien plants, Prosopis spp and Acacia mearnsii, under current and future climate change scenarios across South Africa. The maximum entropy (MaxEnt) model was used with species occurrence data and bioclimatic variables. The Species occurrence data was obtained from the Global Biodiversity Information Facility (GBIF), while the bioclimatic variables were downloaded from the WorldClim database. The model evaluation metrics for training and test samples were the area under curve (AUC) of 0.76 and 0.77 for Prosopis spp, and 0.91 and 0.89 for A. mearnsii, respectively. It showed that MaxEnt performed well in mapping the distribution of both species. Model results indicated that the near-current potential distribution of Prosopis spp and A. mearnsii in South Africa is significant (93.8% and 9.7% of the total land area, respectively). With the projected climate, Prosopis spp showed an inconsistent result across the General Circulation Models (GCMs), projection times and climate change scenarios. However, with respect to the current potential distribution, the geographical ranges of A. mearnsii will significantly contract (by about 75%) due to climate change. Therefore, it is imperative that policy makers, environmental managers and other stakeholders implement integrated management and control strategies to restrict the distribution of Prosopis spp.Item Open Access Short-term effects of tillage systems, fertilisation, and cropping patterns on soil chemical properties and maize yields in a loamy sand soil in Southern Mozambique(MDPI, 2022) Chichongue, Oscar; Van Tol, Johan J.; Ceronio, Gert M.; Du Preez, Chris C.; Kotze, ElmarieSub-Saharan Africa (SSA) agriculture is characterized by dependence on erratic rainfall, inadequate conservation practices, and a decline in soil fertility resulting in low crop productivity. Therefore, conservation agriculture (CA) has been proposed as an alternative to improve soil fertility and productivity. Hence the aim was to investigate the effects of tillage systems, fertilization, and cropping patterns on selected soil chemical properties (pH, organic carbon, total nitrogen, extractable phosphorus, exchangeable cations, and cation exchange capacity) and identify which cropping pattern maximizes stover and grain maize–legume productivity and land use. A two-year (2016/17–2017/18) field experiment in a loamy sand soil was conducted at Nhacoongo Research Station, southern Mozambique. Two tillage systems (conservation (CA) and conventional tillage (CT)), two fertilization treatments (fertilized and unfertilized), and seven cropping patterns (four sole crops and three maize–legume intercrops) were evaluated in a randomized complete block design with split–split plot arrangement and replicated four times. CA practices resulted in significantly higher soil chemical properties and increased stover and grain yields as compared to CT practices, but fertilization demonstrated insignificant effects on soil chemical properties and significant influences on stover and grain yield of maize and legumes. Cropping patterns induced no significant effect on soil chemical properties and either stover or grain yield. Estimated indices like land equivalent ratio (1.18–2.67) and competitive ratio index (0.01–1.72) confirmed the advantage of intercropping against sole cropping. This is largely supported by the estimated values of aggressivity and relative crowding coefficient. Smallholder farmers can therefore benefit by adopting CA.Item Open Access Fire, herbivores, and vegetation type shape soil biochemistry in sodic patches of a semi-arid Savanna ecosystem(MDPI, 2022) Malongweni, Siviwe Odwa; Van Tol, JohanIn the Kruger National Park (KNP), the lower slopes of catenas have open patches referred to as sodic patches. Fire and herbivores are dominant mediators of vegetation in sodic patches. The effect of fire and herbivores on soil properties of sodic patches remains largely understudied. Moreover, the co-existence of trees and grasses and how they influence savanna soils is an important but poorly understood phenomenon in ecology. Therefore, the present study aimed to determine the influence of 20 years of fire, herbivores, vegetation type, and their interaction on soil biochemistry of sodic patches on the Nkuhlu exclosures in the Kruger National Park, South Africa. We found a higher main effect of fire on available phosphorus, cation exchange capacity, and soil organic matter. The presence of herbivores caused an increase in soil exchangeable cations (K+, Ca2+, Na+, and Mg2+), organic matter, cation exchange capacity, and microbial activity. Tree canopies had a higher effect on total nitrogen, exchangeable Ca and Mg, soil organic matter, and cation exchange capacity than open grassland zones. Our results indicate that changes in vegetation structure due to fire and herbivores and their secondary impact on soil properties should be taken into consideration in managing savannas. Moreover, fire and herbivores play an important role in the maintenance of vegetation type (trees and grasses) in sodic patches.Item Open Access Soil organic matter storage in irrigated Tsitsikamma dairy farms with minimum tilled pasture mixtures: case studies(MDPI, 2022) Loke, Palo Francis; Kotze, Elmarie; Du Preez, Christiaan CorneliusIn recent years, pasture production changed from conventional tilled single pastures to minimum tilled mixed pastures in the Tsitsikamma region, South Africa. However, storage of soil organic matter (SOM) under minimum tilled mixed pastures is not yet quantified. This study evaluated SOM indices in the upper 60 cm soil of six-year-old mixed pastures in the Upper (UT) and Lower (LT) Tsitsikamma regions. Soil samples were collected at 0–15, 15–30, 30–45, and 45–60 cm soil layers of five farms (F1, F2, F3, F4, and F5) treated with different rates of fertilizer (NPK) alone and in combination with dairy effluent (DE) and/or poultry manure (PM). Results of this study indicated that there were no significant differences in bulk density, total nitrogen (N), and rate of potentially mineralizable N (PMN) between farms in the UT region. In the LT region, NPK, DE, and PM combinations improved soil C accumulation relative to the soil application of NPK. Higher C/N ratios in the LT region suggested adequate C for microbial energy and maintenance. Integrating manure into minimum tilled pasture mixtures as a replacement for synthetic fertilizers seems to be a feasible option to promote SOM storage, but remains only feasible by applying site-specific management strategies.Item Open Access Characterisation and effects of different levels of water stress at different growth stages in malt barley under water-limited conditions(MDPI, 2022) Bello, Zaid Adekunle; Van Rensburg, Leon D.; Dlamini, Phesheya; Tfwala, Cinisani M.; Tesfuhuney, WeldemichaelMalt barley is typically grown in dryland conditions in South Africa. It is an important grain after wheat, but little is known about its water requirements and, most importantly, how it responds to water stress. Determining when water stress sets in and how malt barley responds to water deficit during its growing season is crucial for improved management of crop water requirements. The objectives of this study were to evaluate the response of transpiration (T), stomatal conductance (SC), and leaf water potential (LWP) to water stress for different growth stages of malt barley and to characterise water stress to different levels (mild, moderate, and severe). This was achieved by monitoring the water stress indicators (soil- and plant based) under greenhouse conditions in well-watered and water-stressed lysimeters over two seasons. Water stress was characterised into different levels with the aid of soil water content ‘breaking points’ procedure. During the first season, at the end of tillering, flag leaf, and milk/dough growth stages, which represent severe water stress, plant available water (PAW) was below 35%, 56%, 14%, and 36%, respectively. LWP responded in accordance to depletion of soil water during the growing season, with the lowest recorded value to −5.5 MPa at the end of the milk/dough growth stage in the first season. Results also show that inducing water stress resulted in high variability of T and SC for both seasons. In the second season, plants severely stressed during the anthesis growth stage recorded the least total grains per pot (TGPP), with 29.86 g of grains. The study suggests that malt barley should be prevented from experiencing severe water stress during the anthesis and milk/dough stages for optimum malt barley production. Quantification of stress into different levels will enable the evaluation of the impact of different levels of stress on the development, growth, and yield of barley.Item Open Access Climate change impacts on temperature and chill unit trends for apple (malus domestic) production in Ceres, South Africa(MDPI, 2021) Tharaga, Phumudzo; Steyn, Abraham Stephanus; Coetzer, Gesine MariaClimate is an essential part of crop production, determining the suitability of a given region for deciduous fruit products such as apples (Malus domestica). It influences the yield and quality of fruits. There is strong evidence of global and regional-scale climate change since the advent of the industrial era. In South Africa, mean surface temperatures have revealed a warming trend over the last century. This study aimed to assess the impact of climate change on temperature and chill unit trends for apple production in Ceres, South Africa. The daily positive Utah chill units (DPCU) model was used as frequent high temperatures can lead to a high negation volume. Historically observed (1981–2010) and future projected (2011–2100) temperatures were obtained from the South African Weather Service (SAWS) and three ensemble members of the Cubic-Conformal Atmospheric Model (CCAM), respectively. The latter employed the RCP8.5 pathway. Linear trends were calculated for temperature and accumulated PCUs for the historical base period. The probability of accumulating specific threshold PCU values for both historical and future periods was assessed from cumulative distribution functions (CDFs). The historical change in minimum temperatures showed no significant trend. Ceres revealed a warming trend in maximum temperatures over the historical period. By the 2080s, the probability of not exceeding a threshold of 1600 PCUs was exceptionally high for all ensemble members. Future projections showed a decline in the accumulated PCUs of 2–5% by the 2020s, 7–17% by the 2050s, and 20–34% towards the end of the 20th century. Based on these results, it is clear that winter chill units are negatively influenced by climate change. The loss in yield and fruit quality of apples due to climate change can negatively impact the export market, leading to significant economic losses for apple production in the Ceres area.Item Open Access Impacts of soil information on process-based hydrological modelling in the Upper Goukou Catchment, South Africa(MDPI, 2022) Smit, Edward; Van Tol, JohanAlthough soils form an integral part of landscape hydrological processes, the importance of soil information in hydrological modelling is often neglected. This study investigated the impact of soil information on streamflow modelling accuracy and hydrological process representation. Two different levels of soil information were compared to long-term streamflow in the upper Goukou catchment (230 km2), South Africa, over a period of 23 years using the Soil Water Assessment Tool (SWAT+). The land-type soil map (LTSM) dataset was less detailed and derived from the best, readily available soil dataset for South Africa currently. The hydrological soil map (HSM) dataset was more detailed and was created using infield hydropedological soil observations combined with digital soil-mapping techniques. Monthly streamflow simulation was similar for both soil datasets, with Nash–Sutcliffe efficiency and Kling–Gupta efficiency values of 0.57 and 0.59 (HSM) and 0.56 and 0.60 (LTSM), respectively. It is, however, important to assess through which hydrological processes were these streamflow values generated as well as their spatial distribution within the catchment. Upon further assessment, the representation of hydrological processes within the catchment differed greatly between the two datasets, with the HSM more accurately representing the internal hydrological processes, as it was based on infield observations. It was concluded that hydropedological information could be of great value in effective catchment management strategies since it improves representation of internal catchment processes.Item Open Access Soil fertilization synergistically enhances the impact of pollination services in increasing seed yield of sunflower under dryland conditions(Cambridge University Press, 2021) Adelabu, Dollop Bola; Bredenhand, Emile; Van der Merwe, Sean; Franke, Angelinus CorneliusTo exploit the potential of ecological intensification during sunflower cropping, it is crucial to understand the potential synergies between crop management and ecosystem services. We therefore examined the effect of pollination intensification on sunflower yield and productivity under various levels of soil fertilization over two seasons in the eastern Free State, South Africa. We manipulated soil fertility with fertilizer applications and pollination with exclusion bags. We found a synergetic effect between pollination and soil fertilization whereby increasing pollination intensity led to a far higher impact on sunflower yield when the soil had been fertilized. Specifically, the intensification of insect pollination increased seed yield by approximately 0.4 ton/ha on nutrient poor soil and by approximately 1.7 ton/ha on moderately fertilized soil. Our findings suggest that sunflower crops on adequate balanced soil fertility will receive abundant insect pollination and may gain more from both synergies than crops grown in areas with degraded soil fertility.Item Open Access Influence of clay mineral amendments characteristics on heavy metals uptake in vetiver grass (Chrysopogon zizanioides L. Roberty) and Indian mustard (Brassica juncea L. Czern)(MDPI, 2022) Otunola, Beatrice Omonike; Aghoghovwia, Makhosazana P.; Thwala, Melusi; Gomez-Arias, Alba; Jordaan, Rian; Hernandez, Julio Castillo; Ololade, Olusola OluwayemisiPhytoremediation is limited when heavy metals reduce soil quality and, subsequently, inhibit plant growth. In this study, we evaluated the use of attapulgite and bentonite as amendments in soil contaminated with multiple metals, to improve the phytoremediation capacity of Vetiver grass and Indian mustard. A 21-day greenhouse study was undertaken, to investigate plant tolerance in heavy-metal-contaminated soil, as well as heavy-metal absorption in plant roots and shoots. The results showed a generally higher root-uptake rate for Cr, Cu, Co, Ni, and Zn in Vetiver grass. Overall, the highest absorption for Ni, Cr, Co, Cu, and Zn was 1.37, 2.79, 1.39, 2.48 and 3.51 mg/kg, respectively, in the roots of Vetiver grass. Clay minerals inhibited the translocation of some heavy metals. The addition of attapulgite improved the phytoremediation capacity of Vetiver for Ni, Cr, and Co, while bentonite improved Vetiver’s absorption of Cu and Zn. The translocation factor for Ni in one of the attapulgite treatments was 2, indicating that attapulgite improved the phytoextraction of Ni by Vetiver grass. Our results confirm that attapulgite at 2.5% (w/w) can successfully improve the phytostabilization of heavy metals by Vetiver grass. Indian mustard showed no significant metal uptake that could be detected by inductively coupled plasma optical emission spectrometry (ICP-OES), despite the addition of attapulgite and bentonite. This research contributes to the knowledge repository of suitable amendments that improve the phytoremediation properties of Vetiver grass.Item Open Access High-density grazing in southern Africa: inspiration by nature leads to conservation?(Sage, 2022) Franke, A. C.; Kotze, E.High-density grazing is a form of rangeland management aiming to strategically mimic the ways grasslands are utilized by grazers in natural situations. It aims to regenerate grasslands by improving soil and vegetation productivity and diversity. More recently, high-density grazing systems have been promoted as a key approach to mitigating climate change by increasing the amount of carbon sequestered in grassland soils. In this article, we describe the historical background of grazing and rangeland degradation in southern Africa, the principles of high-density grazing, and the problems it aims to address. We briefly discuss evidence of the potential benefits of high-density grazing, though we do not aim to provide an exhaustive review on this. We explore to what extent high-density grazing can be regarded as representative of grazing in natural ecosystems and whether the assumed link between nature and high-density grazing has been helpful in capitalizing on the potential merits of high-density grazing. While high-density grazing may represent a form of sustainable rangeland management, the main attractiveness to farmers likely relates to potential increases in livestock densities and associated productivity per unit area, as well as to potential management and social benefits. Learning from nature and inspiration by nature can play an important role in the development and communication of sustainable grazing management systems. However, it is questionable to what extent high-density grazing systems can be seen as more representative of natural ecosystems than other grazing management systems. The claimed ecological superiority of high-density grazing because of its association with nature has polarised and blurred the discussion on the potential merits of high-density grazing. Moreover, the supposed relationship between nature and high-density grazing may have led to an overselling of high-density grazing principles and an embracement of them by policy makers and development agencies without sufficient empirical basis.Item Open Access Using hydropedological characteristics to improve modelling accuracy in Afromontane catchments(Elsevier, 2022) Harrison, Rowena Louise; Van Tol, Johan; Toucher, Michele L.Study region: Three Afromontane catchments in the Cathedral Peak experimental research site, within the uKhahlamba-Drakensberg escarpment, KwaZulu-Natal, South Africa. Study focus: Gaining insight into the hydropedological behaviour of catchments enables a deeper understanding of the unique lateral flow dynamics of a landscape and how these affect the hydrological cycle. This study aimed to highlight the importance of understanding the hydropedological behaviour of soils to improve modelling accuracy. New hydrological insights: Two sets of SWAT+ models were set up for each catchment. The default lateral time, which is the measure of the time required for water to flow through the catchment before being discharged into the stream, was used in the first set up. Specific lateral time inputs, derived from hydropedological soil maps, were utilised in the second model set up and the results compared against observed streamflow. The specific lateral time inputs were based on measured hydraulic properties of the soils coupled with the location of hydrological response units within hydropedological soil maps created for each catchment. The specific lateral time inputs improved modelling accuracy in all statistical parameters used, R2 (i.e., 0.550–0.903), PBIAS (i.e., 19.742–18.239), ST DEV (i.e., 63.42–51.81), NSE (i.e., 0.316–0.864) and KGE (i.e., 0.630–0.807). This study has highlighted that relevant soil information, based on reliable site-specific data, is essential in hydrological modelling.